House wiring optical fibers used for the FTTH (fiber to the home) technique desirably have excellent bending loss characteristics when flexibility of laying cables and ease of installation are taken into consideration. Although improvement in bending loss characteristics can be achieved by increasing a relative refractive index difference between the core and cladding, an increased relative refractive index difference increases containment of higher-order modes, thereby making cut-off wavelengths in higher-order modes longer.
A hole-assisted holey fiber (HAHF) has been proposed as a significantly low-loss fiber compared to typical single-mode fibers (hereinafter abbreviated as “SMFs”) used in an optical transmission path. A holey fiber is constructed such that a plurality of holes are disposed around its core, and it exhibits a larger relative refractive index difference between the core and the cladding and a lower bending loss characteristic even with small-diameter bending, as compared to conventional SMFs.
However, even through conventional holey fibers can achieve low bending loss, they cannot completely eliminate the tradeoff of a lower bending loss versus a longer cut-off wavelength (see T. Hasegawa, et al, “Novel hole-assisted lightguide fiber exhibiting large anomalous dispersion and low loss below 1 dB/km”, Proc. OFC, PD5, 2001).
The following types of hole-assisted holey fibers (hereinafter referred to as HAHFs) have been proposed: one in which a plurality of holes are disposed around the core to form one layer of holes, as shown in FIG. 1 (see B. Yao, et al., “Low-loss holey fiber,” Proc. 53rd IWCS, pp. 135-139, 2004, for example); and another in which a plurality of holes are disposed around the core to form two layers of holes, as shown in FIG. 2 (see T. Hasegawa, et al., “Bending-insensitive single-mode holey fiber with SMF-compatibility for optical wiring applications,” ECOC-IOOC 2003 Proc., We2.7.3, 2003, for example).
The type of HAHF shown in FIG. 1 in which holes are arranged to form one layer includes a core 11, a cladding region 12 therearound, the core region having a higher refractive index than that of the cladding region 12, and a plurality (six in the example shown in the figure) of holes 13 provided in the cladding region 12 along a circle concentric around the core region 11. Each of the plurality of holes 13 has the same diameter, and the distances between the center of the core region and the centers of each of the holes 13 are all equal.
The type of HAHF shown in FIG. 2 in which holes are arranged to form two layers includes a core 11, a cladding region 12 therearound, the core region having a higher refractive index than that of the cladding region 12, six inner holes 14 provided in the cladding region 12 along a circle concentric around the core region 11, and twelve outer holes 15 provided outside from the inner holes 14. In the illustrated example, the inner holes 14 and the outer holes 15 are all formed to have the same diameter. Furthermore, in this HAHF, one-half of the outer holes 15 are arranged on lines extended from the center of the core region through the inner holes 14.
Among the above-described conventional HAHFs, although the HAHF shown in FIG. 1 has a simpler structure than the HAHF shown in FIG. 2, obtaining a fiber with a low bending loss LB and a short cut-off wavelength λC is limited by a trade-off. More specifically, in order to reduce the bending loss LB, the diameter of the holes should be enlarged so that the ratio of the space occupied by the holes in the cladding region is increased. On the other hand, in order to reduce the cut-off wavelength λC, containment of higher-order modes should be minimized. For this purpose, the space occupancy ratio should be reduced or the holes should be disposed closer to the core region. However, in the structure as shown in FIG. 1, since disposing the holes closer to the core region results in an increased space occupancy ratio, it becomes difficult to achieve both a low bending loss LB and a short cut-off wavelength λC.
Furthermore, since the HAHF shown in FIG. 2 has the second layer of the holes outside from the holes in the fiber shown in FIG. 1, it permits more flexibility in adjusting the bending loss and the cut-off wavelength than the HAHF shown in FIG. 1. However, a large number of holes, i.e., 18, and a relatively complex structure of this fiber may cause increased production cost.